Restrict monospaced fonts to serif ones, i.e. Courier family.

I'm not sure it's a win for readability but I'm assuming a serif monospaced font would look better among serif text (and math).
Almost all monospaced fonts are sans-serif; [Courier and derivatives](https://en.wikipedia.org/wiki/Courier_(typeface)) are AFAICT the only serif monospaced fonts that come pre-installed.  (For now I'm not considering web fonts e.g. TeX Gyre Cursor — unicode coverage tends to be low and I don't want the extra loading).

Before: http://www.browserstack.com/screenshots/da632239cca396927592b1aa5de3df569d2646ae
After: http://www.browserstack.com/screenshots/e3459310270061ddca648b82ec50acc0415177fe

- On Windows old and new look OK but new is a better match to surrounding serif.
- On OS X, old was too heavy and high x-height, new is too light with OK x-height.
- On Linux, old was [sometimes not even monospaced?!](http://browsershots.org/screenshots/b521259db3b858d7092dbc89ed263e88)
  but usually [sans, a bit light and too high — probably Liberation Mono replacing Courier](http://browsershots.org/screenshots/820d0ce92f5a322888efefe09b9cc223).
  New is [a bit too light](http://browsershots.org/screenshots/4dae1abf4fb2f8db336faba735501042) and sometimes [too low x-height](http://browsershots.org/screenshots/0fe7f94e8fd8930757580dc070912710).

TODO: Test [Courier 10 Pitch](http://typographyforlawyers.com/courier-10-pitch.html) which should look better (and not so light) on Linux.